Power System Concepts For The Lunar Outpost: A Review Of .
NASA/TM—2006-214248DOE/NV/11718–1118Power System Concepts for the Lunar Outpost: AReview of the Power Generation, Energy Storage,Power Management and Distribution (PMAD)System Requirements and Potential Technologiesfor Development of the Lunar OutpostZ. KhanBechtel Power, Frederick, MarylandA. Vranis and A. ZavoicoBechtel National, Inc., Frederick, MarylandS. FreidBechtel Nevada, Las Vegas, NevadaB. MannersGlenn Research Center, Cleveland, OhioJune 2006
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NASA/TM—2006-214248DOE/NV/11718–1118Power System Concepts for the Lunar Outpost: AReview of the Power Generation, Energy Storage,Power Management and Distribution (PMAD)System Requirements and Potential Technologiesfor Development of the Lunar OutpostZ. KhanBechtel Power, Frederick, MarylandA. Vranis and A. ZavoicoBechtel National, Inc., Frederick, MarylandS. FreidBechtel Nevada, Las Vegas, NevadaB. MannersGlenn Research Center, Cleveland, OhioPrepared for theSpace Technology and Applications International Forum (STAIF–2006)sponsored by the University of New Mexico’s Institute for Spaceand Nuclear Power Studies (UNM-ISNPS)Albuquerque, New Mexico, February 12–16, 2006National Aeronautics andSpace AdministrationGlenn Research CenterCleveland, Ohio 44135June 2006
AcknowledgmentsZahid A. Khan thanks Jeffrey Hojnicki and Thomas Kerslake (NASA Glenn Research Center) for their editorial comments andreview. Thanks to Paul Nastick and Ram Narula (Bechtel Power) for their support of this paper. This manuscript has beenauthored by Bechtel Nevada under contract DE–AC08–96NV11718 with the Department of Energy.This report contains preliminary findings,subject to revision as analysis proceeds.Level of Review: This material has been technically reviewed by technical management.Available fromNASA Center for Aerospace Information7121 Standard DriveHanover, MD 21076–1320National Technical Information Service5285 Port Royal RoadSpringfield, VA 22161Available electronically at http://gltrs.grc.nasa.gov
Power System Concepts for the Lunar Outpost: A Review of thePower Generation, Energy Storage, Power Management andDistribution (PMAD) System Requirements and PotentialTechnologies for Development of the Lunar OutpostZ. KhanBechtel PowerFrederick, Maryland 21703A. Vranis and A. ZavoicoBechtel National, Inc.Frederick, Maryland 21703S. FreidBechtel NevadaLas Vegas, Nevada 89193B. MannersNational Aeronautics and Space AdministrationGlenn Research CenterCleveland, Ohio 44135AbstractThis paper will review potential power system concepts for the development of the lunar outpost includingpower generation, energy storage, and power management and distribution (PMAD). In particular, the requirementsof the initial robotic missions will be discussed and the technologies considered will include cryogenics andregenerative fuel cells (RFC), AC, and DC transmission line technology, high voltage and low voltage powertransmission, conductor materials of construction and power beaming concepts for transmitting power to difficult toaccess locations such as at the bottom of craters. Operating conditions, component characteristics, reliability,maintainability, constructability, system safety, technology gaps/risk and adaptability for future lunar missions willbe discussed for the technologies considered.IntroductionA power supply system whether terrestrial or space based can be divided into four (4) major sub-systems: Power GenerationEnergy StoragePower Management and DistributionLoadsPrior work published on this subject includes (Cataldo and Bozek, 1993; Kerslake, 2005). This paper will focuson the power requirements of the early development stages of the lunar base. The paper assumes a lunar base sited atthe South Pole near the Shackleton crater and that power generation will be based on an initial photovoltaic andregenerative fuel cell (PV/RFC) power source. Figure 1 illustrates a notional Phase 0 base layout. High-level tradestudies for energy storage and power management and distribution (PMAD) systems are presented and potentialtechnology gaps/risks are identified.Lunar power supply systems are optimized primarily on mass, in order to reduce launch costs from earth. Theprimary quantitative metric used in this paper will be energy density, kWh/kg.NASA/TM—2006-2142481
Figure 1.—Notional phase-0 base layout.A qualitative assessment of constructability, maintainability, reliability, adaptability, system safety andtechnology gaps/risk will also be made and will use the following ranking criteria in table 1:Category versus ptabilitySystem SafetyTechnology Gaps/RiskTABLE 1.—TECHNOLOGY RANKING CRITERIA12Complicated structuralrequirementsHigh maintenance frequencywith earth based logisticalsupportLow reliability with a largenumber of moving partsIntermediate structuralrequirementsIntermediate maintenancefrequency with earth basedlogistical supportIntermediate reliability withsome moving partsLow adaptability to Marsand Beyond missionsHigh level of risk for loss oflife, equipment or propertyProof of conceptdemonstratedIntermediate adaptability toMars and Beyond missionsMedium level of risk for lossof life, equipment or propertyDemonstrated in terrestrialenvironment3Simple structuralrequirementsRelatively low maintenancefrequency with earth basedlogistical supportHigh reliability with aminimum number of movingpartsGood adaptability to Marsand Beyond missionsLow level of risk for loss oflife, equipment or propertyDemonstrated in spaceenvironmentThe primary trade studies that are investigated in this paper include the following: Regenerative Fuel Cell (RFC) versus Batteries versus Flywheel energy storage systemsAC versus DC transmissionHigh voltage versus Low voltage transmissionTransmission frequency, materials of construction, conductor placement, and power transmission into difficultto access areas are also discussed.The following overview of terrestrial based and space based power systems is provided as a brief summary ofthe current state of power system technology.NASA/TM—2006-2142482
Terrestrial Power SystemsIn the United States, power generation is based on a centralized architecture with 3 Phase, 60 Hz AC powerbeing generated at medium voltage, generally between 11 and 25 kV. The generated voltage is then stepped up tohigh voltage levels (typically 115 and 230 kV) or ultra-high voltage levels (typically 345 kV, 500 kV, and higher)for transmission. The distribution system is split into two categories – primary and secondary. Primary voltages areusually medium voltage systems between 5 and 69 kV. Secondary voltages are usually single phase 120/240 VACor 3-phase 120/208 VAC or 480 VAC.Terrestrial power generation is characterized by a wide range of fuels that includes coal, natural gas, nuclearand renewables. This adds diversity to the overall system and results in greater system flexibility and reliability.Recent trends in terrestrial power generation include cogeneration and distributed generation. Cogeneration unitsgenerate both electricity and heat and results in a very efficient form of generation. Distributed generation can bedefined as locating the generating source near the consumers site and can be integrated or isolated from the maingrid. Distributed generation appears to have the ability to provide higher quality and higher reliability power thancentralized power stations but still needs additional technology improvements in order to bring costs down.The transmission system in the U.S. has evolved in to three regional grids interconnected for reliability.Aluminum conductors and buses are the norm for most terrestrial transmission lines, open-air switchyards,switchgear and motor control centers. The operational capacity of the system is determined by thermal constraints,voltage constraints and system operating constraints.Space Based Power SystemsSOLARARRAYThe International Space Station (ISS) Electrical Power System (EPS) consists of a 120 VDC U.S. built portionand a 28/120 VDC Russian made section. The two sections are interconnected and allow mutual transfer of power.Both systems use PV arrays to generate power during the sunlit portion of the orbit and batteries for the eclipseportion. The U.S. EPS provides approximately 78 kWe of power and the Russian EPS provides approximately29 kWe (Gietl et al., 2000). Details of the U.S. EPS follow as this system most closely matches the requirements forthe initial Phase 0 PMAD. Figure 2 illustrates the U.S. EPS power flow.P rim a ryD istribu tio n13 8-17 3 V d c12 0-12 6 V d c12 6-17 3 V d cB eta48 .3 kW G im b alS SU21 .9 kWA lph aG im b alSOLARARRAYB C DU21 .0 kW19 .1 kWM BSUD C SU23 .3 kWS ec on da ryD istribu tio n21 .9 kW23 .9 kWDDCUDDCU26 .6 kWR u ssianS ou rc es Am e ric an -to-R uss ian C on ve rte r U n it Ba ttery C h a rge /D is ch arge U nit D c S w itchin g U n it D c-to -D c C o n ve rs ion U nit M ain Bu s S w itchin g U n it R u s sia n-to -A m e rican C on ve rte r U n it R e m ote P o w e r C o ntro lle r M od u leS e q ue n tial S hu n t U nitRAC UARC UARC UToR u ssianLo ad sRPC sToU .S . Lo ad sSunlit Period54.9 (minimum)Eclipse Period(36.5 (maximum)Figure 2.—ISS single channel power flow diagram (Gietl et al., 2000).NASA/TM—2006-214248U se rs19 .1 kWBATSAR C UBC D UDC SUDDCUM B SURACURPCMSS U RP CM35 F EB 9 9FILE: EPS-DIST .DSFG HO L DS T ON /JLM
The U.S. EPS provides 78 kWe of power generated by four photovoltaic array modules. Each Array consists oftwo flexible deployable array wings of silicon solar cells supported by an extendable mast. This results in a total ofeight (8) solar array panels. During an eclipse Nickel-Hydrogen batteries provides station power. The PMAD systemdistributes power at 160 VDC using DC contactor switchgear. This voltage is then stepped down to 120 V DC byDC-DC converter units that condition the voltage for use by end users through solid-state switchgear (Ianculescu,1999).In the sunlit portion of the orbit, power flows from the solar array through the Sequential Shunt Unit (SSU) tothe DC Switching Unit (DCSU). Power then flows downstream to loads connected to the DC-DC Converter Units(DDCU) and to the batteries through the Battery Charge Discharge Unit (BCDU). During eclipse periods and duringsun periods with low solar array output, power flows out of the batteries through the BCDU to the DCSU whichroutes power to the connected loads. The station uses two redundant channels for power distribution. Thetemperatures of the batteries and electronics are maintained via an active thermal control system.General Design Considerations for a Polar SiteThis paper assumes a lunar base sited at the South Pole near the Shackleton crater. The relatively constantthermal and illumination conditions along with the potential of water at the South Pole makes this area an attractivesite for a manned lunar base. Preliminary data indicates that regions located near the Shackleton crater receivesunlight for much of the lunar day (Cook et al., 2000).Estimated lunar temperatures at polar sites averages 40 K in shadowed polar craters and approximately 220 K inother polar areas with a monthly range of 10 K. The thermal conductivity of the sub-surface lunar soil is on the orderof 1.4 to 3.0 10–4 W/cm /K. The electrical properties of the lunar soil are characterized by extremely low electricalconductivity. In the absence of water and in darkness, the DC electrical conductivity is in the range of 10–09 to10–14 mho/m. The low conductivity of the lunar soil also contributes to the fact lunar materials are chargeable andremains charged for long periods of time. Charged surface soil particles can levitate and move and coat solar arraysor radiators (Eckart, 1999). This may result in a decrease in power system performance.Lunar Base Phased ConstructionWe are assuming that the construction of a lunar base and its supporting infrastructure will undergo anevolutionary development. The infrastructure must allow for increased power, mobility and research capability ateach stage. The construction of the lunar base may be undertaken in five (5) phases over a period of 15 to 20 years.The final phase, Phase 4, will involve self-sufficient scientific and commercial projects with only discretionary linksto the earth (Duke and Freid, 2004). Phase 0—Robotic Site Preparation (minimum or no human presence)Phase 1—Deployment and initial operations (3 to 4 personnel for 4 to 6 months)Phase 2—Growth Phase (approximately 10 personnel for a year)Phase 3—Self Sufficiency (ten to 100 personnel for extended periods)Phase 4—Science and Commercial (greater than 100 personnel for unlimited durations)Lunar Surface Power Supply RequirementsPhase 0 mission requirements includes robotic exploration with landers and rovers with minimum or no initialhuman presence. The landers and rovers will survey the lunar terrain, test accessibility to permanently shadowedareas, demonstrate basic construction techniques, demonstrate propellant production, confirm the existence of water,and establish and demonstrate power system operations.An estimate of the array peak power requirements for Phase 0 is listed in table 2. The highest expected load of43 kWe is for In-Situ Resource Utilization (ISRU) mining activities for water, hydrogen and oxygen (Blair, 2005).Additional loads are included for general science activities and rover recharging. Parasitic loads for the RFCelectrolyzer are based on 65 percent sunlight and 50 percent nighttime power. Emergency power for equipmenthealth monitoring and thermal control may be provided by an independent power source such as batteries.Additional base power requirements for subsequent phases can be added in a modular fashion.NASA/TM—2006-2142484
TABLE 2.—ESTIMATED ARRAY PEAK POWER REQUIREMENTS FOR PHASE-0LoadsISRU mining (Blair, 2005)General Science (Cataldo and Bozek, 1993)Rover Recharging (Catadalo and Bozek, 1993)RFC Parasitic Load10 percent marginTotalPhase 0kWe431525781Power GenerationPortable power in the 1-2 kWe range for rovers and landers can be provided by a number of different options.Power may be provided by Radioisotope Thermal Generators (RTG) with batteries for peaking power or an RFCsystem (Kerslake, 2005).The proposed PMAD architecture would be a hybrid design with centralized power from a PV/RFC source forinitial ISRU loads and emergency power provided by batteries. This would also provide dissimilar redundancy.Power could be generated at the array panels at relatively low voltages (200 VDC) for better PV componentreliability and then conditioned to higher voltages for transmission.PV technology is used extensively in space-based systems and is currently being used for power generation onthe ISS. PV power generation takes advantage of the relatively constant illumination at the Shackleton crater toconvert free solar energy into electricity. However, like all solar powered systems an energy storage system will berequired for nighttime power. This can be provided by an RFC system.In order to meet future multi-MWe power requirements, a nuclear fission reactor may be required for laterstages. The trade between PV and Nuclear Reactor power is not considered here.Energy StorageEnergy storage options include RFC’s and batteries. Energy storage using flywheels, thermal reservoirs,supercapacitors and gravitational fields is also possible although they are not as mass efficient. Flywheel energystorage systems do have the potential for a high lifetime, improved energy density, power and voltage levels.Fuel cells combine hydrogen and oxygen to produce electrical energy, water and waste heat. A fuel cell can becoupled with an electrolyzer in an RFC arrangement in order to regenerate the reactants. This system would functionas a fuel cell at nighttime and regenerate the reactants during the day. The additional power requirements needed torun the electrolyzer during the day needs to be included in the base load power requirements.Cryogenic RFC systems store the reactants as liquids in tanks. Cryogenic reactant storage increases the energydensity of the overall system by reducing the volume of the stored reactants and the mass of the storage tanks.Permanently shadowed areas exist in certain craters at the South Pole and may allow for cryogenic storage ofreactants at these locations. This would result in reduced liquefaction equipment load although additional equipment(pumps and/or transfer rovers) would be needed to transport the fuel to the land and leave area.Primary batteries are defined as non-rechargeable and do not have sufficient energy density to be considered forlunar base use. Secondary batteries are defined as rechargeable and can be used in a variety of space-basedapplications including lunar landers, rovers, human outposts and astronaut equipment. Nickel-hydrogen secondarybatteries are currently in use on the ISS. However these batteries have limited storage capacity and are not suitablefor lunar base application. Lithium-ion battery technology offers higher specific energy with the capability ofoperating at higher voltages over a wide range of temperatures.Table 3 summarizes the basic characteristics of RFC, battery and flywheel energy storage. Battery depth ofdischarge is assumed as 80 percent. Energy density values are representative of current technology (Eckart, 1999).RFC energy density is based on cryogenic storage.NASA/TM—2006-2142485
TABLE 3.—RFC VERSUS BATTERIES VERSUS FLYWHEELSCriteriaRFC (cryogenic)BatteriesNighttime Stored Energy, kWh6,4498,062Energy Storage Mass, kg4,30080,617Energy Density, lityAdaptabilitySystem SafetyTechnology 80.0322222111Power Management and DistributionThe PMAD consists of power conditioning equipment, switchgear and high voltage transmission lines. ThePMAD system mass is governed by the PMAD architecture, the output power demands, the power source,transmission voltages, operating frequencies, stage efficiencies, transmission line design, and materials ofconstruction.Space based power supply systems have evolved from the small satellite requirements of less than 1 kWe,where power consumption is in close proximity to the power consumers, to systems like that of the ISS wheremultiple sources, multiple loads and large physical size make the ISS power system more like a terrestrial utilitysystem than a traditional spacecraft power system.The requirements of the Phase 0 lunar surface PMAD are very similar to the ISS EPS. Both systems arecharacterized by multiple sources, multiple users, similar power generation levels and similar secondary distributionvoltages. Important differences include transmission distances in the 10’s of km’s and that loads will be mobile andfixed for the initial Phase 0 lunar base. It should also be noted that during the development of the lunar base thepower supply system will have to evolve from the relatively low power requirements (multi-kWe) of the initialstages to the much higher power requirements (multi-MWe) of the later stages along with the possibility of evengreater transmission distances. This may require a nuclear reactor power source for later stages.AC versus DC transmission.—DC power transmission is typically more efficient than AC because DC linelosses are primarily due to conductor resistance. For AC transmission, “skin effects” (uneven current densityresulting in greater effective resistance) plus reactance terms must be added to the conductor resistance. Because ofthese differences AC and DC transmission need different cable configurations. DC lines are simpler to fabricate. ACconductors require specialized construction in order to minimize reactance terms and skin effects. AC has theadvantages of simpler voltage transformations and the ability to switch on zero current which simplifies powerconditioning component design. The trade between AC and DC transmission is not clear cut and more detailed tradestudies need to be done that take into account safety and life cycle costs.As noted in the previous section, the power supply requirements of the Phase 0 lunar base PMAD are similar tothe ISS PMAD except for longer transmission distances. It may be possible to install an ISS derived, DC basedPMAD for Phase 0 that could be adapted for the relatively short distance of high voltage transmission. This wouldtake advantage of the ISS EPS’s developmental work and allow for quicker flight testing of new high voltagecomponents. Initial PMAD designs must be able to evolve and adapt to future requirements that may include ACpower generation. If a DC based system is selected for the initial Phase 0 PMAD this system could be adapted tointerface with an AC system with solid-state inverters. Additional trade off studies will have to be performed toassess the mass and cost penalty associated with this.Table 4 summarizes the trade between AC and DC transmission. DC transmission systems are expected to berelatively easy to install and maintain. DC systems are given a slightly lower ranking for system safety because ofthe greater potential of arcing at switchgear. This may be addressed with DC PMAD architectures involving lowvoltage DC switchgear and high voltage side current limiting protective devices. Additional studies involvinganalysis, testing and prototyping of suitable DC switchgear technology for reducing arcing and high fault currentsneed to be undertaken. Although terrestrial transmission is based on AC transmission, there are currently no spacebased applications. All existing space based electrical power systems are DC.NASA/TM—2006-2142486
CriteriaAdvantages(Metcalf, Harty, andRobin, 2001) Disadvantages(Metcalf, Harty, andRobin, ptabilitySystem SafetyTechnology Gaps/RiskTotal TABLE 4.—AC VERSUS DC TRANSMISSIONACDCVoltage transformations less complex Transmission lines are more efficientSwitching on zero current eases fault Easier to parallel DC linescurrent interruption Low voltage DC used developed forSwitching on zero current reducesSSFswitching losses, transients and EMIIncreased transmission line losses due to Voltage transformations require 3 stepsskin effect High fault currents, difficult tointerrupt, damaging arcFurther R&D in paralleling sourcesAC to AC frequency conversion requires 2 Channelized approach probablystepsnecessary to handle fault currents,reduces power utilization efficiency3223321533332317It may be noted that for terrestrial applications, there has been a recent trend in the electric power transmissionindustry to utilize high voltage DC (HVDC) transmission lines at different voltage levels for bulk powertransmission over long distances. There are numerous HVDC projects with voltage levels up to 500 kV alreadyinstalled and operating around the world presently, which provide valuable field lessons learned to the industry. Inaddition, large HVDC transmission schemes over distances between 1000 and 2000 km are currently being plannedfor various hydroelectric power stations. It appears that ultra high DC voltage in the range of 800 kV is the preferredvoltage for this application. This planned ultra high DC voltage scheme is being designed to be bipolar,unidirectional (although power reversal shall be also possible), with ratings exceeding 6000 MW. In general, withthe HVDC technology in terrestrial applications, the installed systems presently are being used not only to transmitpower over extremely long distances, but to also connect to AC asynchronous grid systems, i.e., AC systemsoperating at different frequencies.High voltage versus low voltage transmission.—High voltage transmission is defined in this paper as voltagesgreater than 300 V. Transmission line voltage affects component sizing in two ways. Increasing the voltage levelreduces the amount of current a line must carry. This reduces the conductor mass and increases transmission lineefficiency. A higher voltage also requires more insulation and greater separation distances which increases hardwarevolume and mass. On a system level, previous studies have shown that power system mass approaches a minimumvalue of 5000 V for both AC and DC systems (Metcalf, Harty, and Robin, 2001).ISS transmission voltages are approximately 160 VDC and would not be optimum for transmitting power over10’s of km’s. It would be necessary to step up the voltage to a higher voltage level in order to minimize I2R lossesand increase transmission efficiency. This transmission voltage would be the result of another more detailed tradestudy.Table 5 summarizes the trade between high and low voltage transmission. High voltage transmission isexpected to be relatively easy to install due to its lower overall system mass, have a low maintenance frequencybased on terrestrial experience, and a high adaptability to other surface based power systems. Although terrestrialtransmission is based on high voltage transmission, there are currently no space based applications aboveapproximately 160 V. High voltage systems are considered to pose a greater safety hazard than low voltage systems,and this will need to be addressed with additional analysis, testing and prototyping.NASA/TM—2006-2142487
CriteriaAdvantages(Metcalf, Harty, and Robin,2001) TABLE 5.—TRANSMISSION VOLTAGEHigh Voltage(Greater than 300 V)Reduced Transmission Line Mass Increased Efficiency Disadvantages(Metcalf, Harty, and Robin,2001) litySystem SafetyTechnology Gaps/RiskTotalAt equivalent efficiencies linetemperatures risePower Conditioning requires moreinsulationAdditional testing of high voltageswitches33332216 Low Voltage(Less than 300 V)Existing Space Based DesignsDC to DC converters easier tomanufacture and probably morereliableMinimal Electrical Insulation forPower ConditioningDC to DC conversions less efficientHigh I2 lossesHigh switching losses22223314PMAD frequency.—In an AC system, as frequency increases the mass of the power conditioning componentsdecrease, and the transmission line mass increases. The mass of the power conditioning equipment decreases withfrequency because of a decrease in the required flux density for a specified voltage. Transmission line massincreases with frequency as skin effect and inductive reactance increase with frequency. Previous studies indicatethat the reduction in mass of the PMAD system due to increasing frequency has a relatively minor effect as thepower source mass dominates the overall system (Metcalf, Harty, and Robin, 2001).Equipment and conductor materials of construction, conductor placement, and grounding.—Temperaturesexpected in shadowed areas at the poles are approximately 40 K. This increases to approximately 220 K in otherpolar areas. This will require material of construction for equipment and wiring to be qualified to operate throughoutthis wide temperature range. Materials used in cryogenic service on earth in this temperature range include stainlesssteels and aluminum. Composite materials have also been used in cryogenic service but will need further testing andanalysis to see if they can be used at these temperatures.As the mass of an aluminum conductor is approximately half that of an equivalent Copper conductor,Aluminum is more suitable for external conductors. Aluminum is used exte
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